In our previous patent (U.S. Pat. No. 4,006,260) we presented a thorough review of the art in which the present invention lies. To produce crisp puffed hygroscopic fruit and vegetable particles in an attractive and appetizing form, thin slices such as of apple, pineapple, ripe banana, onion, pumpkin and whole seedless grapes and the like, are subjected to contact with heated edible oil in a vacuum environment until the particles are dehydrated and puffed; the puffed structure is hardened by cooling with cool oil, is moved from the drying region into a centrifuge, and is centrifugally deoiled while still subject to vacuum, is then restored to normal atmospheric pressure to await packaging and shipment.
In U.S. Pat. No. 2,587,939 I describe the earliest version of this method of producing fruit chips. Here, the wet prepared particles were loaded into deep trays at a place outside of the vacuum chamber; the loaded trays were transferred to a vacuum chamber and inserted therein; and after processing was completed the vacuum chamber was opened up and the trays were removed to a location where they could be inverted and emptied.
Certain products, such as soft ripe banana slices had a tendency to stick to some of the tray surfaces and hand labor was required to clean the trays. For this and other reasons there was an incentive to reduce the number of trays by increasing the depth and hence the volume of each tray. On the other hand, tray depth could not be increased beyond a certain practical limit for the following reason. A tray depth of 16 centimeters can be used in the example. It is desired to evaporate moisture from a wet fruit particle that lies 16 centimeters or more beneath the surface of oil in the containing tray, and reach a moisture content in the particle of 2% when the surface of the oil is under a pressure of 4 torr. The particle will then be in a pressure environment of 16.times.10/15.7+4=14.2 torr, where 15.7 is the ratio of the density of mercury to the density of the oil. Moisture will evaporate from the particle so long as the vapor pressure of moisture in the particle exceeds 14.2 torr and the vapor pressure depends upon temperature; but the temperature cannot be raised above a certain point without undesired caramelization of the fruit sugars occurring. If, for example, the temperature of the particle has been raised by heated oil to the limiting temperature before caramelization occurs, and the particle has not been dried to the desired 2% moisture content, and further bubbling of water vapor out of the particle has stopped, then the only way to induce the resumption of drying without raising the temperature further, is to continue the treatment with further reduction of pressure. However in this example the vapor head pressure of 4 torr that exists on the surface of the oil covering the particle, is the limiting low pressure, and further reduction of this pressure would be economically unfeasible. Nevertheless therre is a way by which the pressure on the particle can be further reduced, and this is by providing a shallower tray so that the particle is raised closer to the oil surface and hence is not subjected to so high a measure of static head of oil. Therefore shallow trays of say, 8 centimeters are required to dry this kind of fruit particle, which will double the number of trays required, and double the cleaning duty. The present invention provides for automatic mechanical emptying, cleaning and refilling of the trays and thereby makes possible the economical employment of shallow trays of optimum depth.
The above example demonstrates an exact method for determining optimum tray depth for each kind of fruit particle and drying condition. In sum, the method requires a simple preliminary test using a glass flask that contains heated oil and a representative wet particle of the fruit whose drying characteristics are being tested. The oil is heated to a temperature just below the caramelization temperature of the fruit, and the surface of the oil is kept evacuated to the pressure that has been previously determined to be the economical measure of vacuum. The oil depth beneath which the particle will stop giving off water vapor bubbles when the particle has 2% moisture content or some other desired end point of moisture content, is determined by moving the particle up or down through a range of positions beneath the oil surface. That position of the dehydrated particle at which its water bubbles stop evolving is a measure of the maximum optimum tray depth and establishes the vapor pressure for that particle.
The caramelization temperature of the fruit particle is readily determined by placing a dehydrated particle in cool oil and slowly raising the oil temperature until a slight darkening is observed; this is the caramelization temperature of that particular fruit.
Another difficult encountered in previous attempts to produce these dehydrated puffed fruits lay in the requirement of an adequate quantity of oil chilled below about 35.degree. C. for the cooling of the hot puffed particles to preserve their puffed structure by hardening them just prior to restoring normal atmospheric pressure. This difficulty of providing chilled oil arose from the circumstances under which heat exchangers must operate; for the heat-transfer coefficient in any heat exchanger that is designed to cool available oil that has been previously warmed by use in the process, is extremely low at low oil temperatures. Thus an extremely large surface area of heat exchanger must be used, often in conjunction with a refrigerated cooling fluid such as iced brine. In the present invention I eliminate the need for such expensive and elaborate oil chilling means.
In our U.S. Pat. No. 4,006,260 we introduce another new feature; the centrifuging of the cooled, dehydrated particles to remove excess oil from them before they are discharged from the vacuum environment. The centrifuge was arranged to operate inside of the vacuum processor, but it required the dehydrated particles to be moved out of the dehydrator and into the centrifuge, a difficult step that is eliminated in the present invention.
I have now invented improvements that have overcome the aforementioned difficulties and other difficulties, which will now be described.